1. Field of the Invention
The present invention relates to a propeller for watercraft and an outboard motor.
2. Description of the Related Art
An outboard motor can be attached to a boat body by being simply engaged onto the stern of a boat, and does not occupy any space inside the boat. Therefore, outboard motors are widely used for small-sized boats, e.g., pleasure boats and small fishing boats. In accordance with the boat body sizes and purposes, outboard motors of various output powers are in use today.
Generally speaking, an outboard motor having a propeller made of stainless steel and an engine with high output power (e.g., 100 horsepower or more) is used for a relatively large boat. On the other hand, for a relatively small boat, an outboard motor having a propeller made of aluminum or the like and an engine with relatively low output power is used. An aluminum propeller is light-weight and can be produced at low cost, and therefore is suitable as a propeller of an outboard motor having an engine with low output power.
In the case of forming such a watercraft propeller from aluminum, it is necessary to prevent corrosion of the aluminum alloy caused by seawater. Therefore, generally speaking, propellers having its aluminum-alloy propeller body coated or painted with a corrosion resistance or preventative material are widely used.
Japanese Utility Model No. 3029215 discloses, in order to prevent deteriorations in water dissipation during the rotation of a propeller (which may happen when the propeller edge is made dull by any painted film that is provided on the propeller surface), subjecting an aluminum-alloy propeller to a hard anodized aluminum treatment to secure a sharp propeller edge.
Small-sized boats with outboard motors are often used at inshore locations and on rivers, for purposes such as fishery, business operations, and leisure activities, and may be pulled onto a sand beach for mooring, or may be moored in the shallow sandy area by a river shore. Therefore, when mooring a boat, or when going out onto the river or the sea from a point of mooring, sand may be stirred up, and the propeller surface is likely to be abraded as the propeller is rotated in the sand-containing water. As a result, the paint on the propeller surface peels due to such abrasion, the propeller body may be corroded, and the propeller body may be abraded. A painted coating lacks sufficient hardness, thus resulting in a problem in that the propeller of a conventional outboard motor has a short life due to abrasion.
Japanese Utility Model No. 3029215 merely discloses forming an anodized aluminum layer (which is known as a corrosion-protective coating for aluminum), instead of a painted film for corrosion protection, without teaching the aforementioned problems. Moreover, in order not to allow the propeller edge to become dull, it would be impossible to form a thick layer of hard anodized aluminum. Therefore, the thickness of the hard anodized aluminum layer for a propeller according to Japanese Utility Model No. 3029215 can only be about 15 μm, which is not considered to provide sufficient abrasion resistance and deformation resistance.
Such problems have occurred with respect not only to boats having outboard motors, but also to small-sized boats having an engine installed inside the boat.